Conventional media milling uses a media that is denser than the fluid dispersion being milled which makes separation of the media and dispersion relatively easy. Under the influence of centripetal force, the denser media disproportionally populates the outer regions of the mill as the agitator rotates allowing the media-free dispersion to escape through the mill center under positive pressure. In the center is a small screen which ideally never encounters media as it is fragile and expensive to replace. The screen is primarily in place to prevent media discharge mishaps during startup/shutdown and stop the occasional stray media from leaving the milling chamber.
When the media and the dispersion are close in density, centripetal force no longer works efficiently as a separation method. This is generally the case when polymeric media is used. For this reason, polymeric media hasn't seen the wide application of ceramic media even though it has many compelling attributes such as increase energy efficiency, reduced mill wear, reduced metal contamination, and often superior particle size reduction at the same energy or throughput.
Tank or batch processes for creating dispersions with polymeric media require large quantities of media to be premixed with the pre-mix. After milling and media-dispersion separation, large quantities of dispersion-laden media remain. This media either needs to be cleaned or stored until a similar product is made again. Each time a product is changed, the media must be cleaned, which is not only laborious but also wastes 20-40% of the dispersion that clings to the media. Storing the dispersion-laden media in a warehouse for the next time the product is made requires a complex logistical plan, and additional chemicals must be used to prevent fungal and bacterial growth plus other potential contaminations. In a tank process, the batch size is limited because large tanks are required to hold the high media content dispersion-media mixes. Large tanks must be assembled on site rather than efficiently mass manufactured. There are also practical limitations to the size of a rotor stator or other high shear device regardless of tank size. A tank process is inherently a batch process which involves a milling step followed by a separation step.
Consequently, a significant need exists for ways to make polymeric media milled dispersions continuously that use small amounts of media which results in less dispersion waste and eliminates the storage/logistic and bacteria growth problems.